Television signals are transmitted at radio frequencies (RF) using terrestrial, cable, or satellite transmission schemes. Terrestrial and cable TV signals are typically transmitted at frequencies of approximately 57 to 860 MHz, with 6 MHz channel spacings. Satellite TV signals are typically transmitted at frequencies of approximately 980 to 2180 MHz.
Modern television receivers may receive the transmissions directly from antenna or from cable networks. More recently the transmissions are modulated so as to transport digital signals representing audio and video signals of a programme. When digital signals are transmitted, different modulation schemes are used for transmissions to be received directly from antenna, so called off-air signals or terrestrial signals, and transmissions received via a cable network, so-called cable reception. Off-air signals are, for example, modulated in a vestigial side-band modulating scheme, often referred to by its acronym VSB. The Advanced Television Standard Committee ATSC stipulated, for the United States, the use of a particular vestigial side-band modulation scheme: 8VSB. 8VSB is an 8-level vestigial side-band modulation. For cable reception of digital television signals a different modulation scheme is used: QAM. QAM is an acronym for Quadrature Amplitude Modulation. QAM is a combination of amplitude modulation and phase shift keying. Television apparatus must be capable to operate from both signal sources, off-air and cable network. Different modulating schemes, such as 8VSB and QAM, have different requirements as to input sensitivity, image rejection, linearity and pass band flatness of a tuner. 8VSB, e.g., requires a high input sensitivity and large values for the image rejection. On the other hand, QAM used for cable network reception requires good linearity and high flatness of the bandpass filter in first place.
In general, image rejection and linearity are two key performance parameters for RF down-conversion. Image rejection and linearity requirements depend on the specific application and the corresponding display requirements, as discussed above. Because of its importance, image rejection is explained further as follows.
FIG. 1 illustrates an example frequency spectrum 100 that illustrates down-conversion and image rejection as performed by a tuner. More specifically, the spectrum 100 illustrates an exemplary RF input having an RF carrier 110 at 800 MHZ, an RF carrier 114 at 888 MHZ and a local oscillator 112 at 844 MHZ, respectively. For purposes of illustration, the local oscillator input is set to 844 MHZ so that the RF input 102 is frequency translated to 44 MHz by the tuner. Assuming no image rejection, the translated signal 114′ falls into the translated IF signal 110′ as shown. Image rejection is calculated as the relative amplitude of the desired image compared to the undesired image. For example, if the channel 110′ is the desired channel, then the image rejection of the tuner is the ratio of the signal 110′ amplitude compared to the signal 114′ amplitude. If the tuner had perfect image rejection, then the amplitude of the signal 114′ would be zero.
Regardless of the transmission scheme, a tuner is utilized to down-convert the received RF signal to an intermediate frequency (IF) signal or a base band signal, which is suitable for processing for display of transmitted content on a TV or computer screen. The tuner should provide sufficient image rejection during down-conversion as is necessary for the specific application. To process a terrestrial television signal, it is preferable that the tuner has a high level of image rejection, whereas for cable reception a high pass band flatness of the filters may be required. The requirements are, however, depending on the modulation scheme used.
State of the Art single conversion tuners provide sufficient pass band flatness for cable reception. An exemplary single conversion tuner 200 according to the prior art is shown in FIG. 2. In the figure, the signal received from a cable network or an antenna covers a wide range of frequencies. The wide range of frequencies is split into three frequency bands B1, B2, B3. This may be necessary since the tuning range of the tuning circuit of one single tuner may not be sufficient to tune the whole range of input RF frequencies. The wide input range may be split into only 2 bands or into a larger number of bands, depending on the application. However, if the range of frequencies in which RF signals are to be received is narrow enough the band select filter 2 may be omitted. For band splitting the RF signal is typically fed to band select filters 2, which are used to split the rather large range of possible RF frequencies coming from the antenna into a number of paths B1, B2, B3 each covering a smaller range of frequencies. From the band select filters 2 the signal is fed to variable gain amplifiers 3. The variable gain amplifiers 3 are used to provide a suitable signal level to the downstream connected processing stages, in order to prevent overdriving the downstream connected processing stages. Single conversion tuners typically have tuneable filters 4 upstream of mixers 6. The tuneable filter 4 is used to select a desired frequency for down-mix in the mixer 6 and to reject the image signal of the tuned channel. The tuneable filters 4 must have a rather large pass band bandwidth while providing good pass band flatness. Therefore, the filters are often implemented as double-tuned tuneable filters. Single-tuned tuneable filters typically do not exhibit sufficient flatness in the pass band. This excludes the use of single-tuned tuneable filters for certain applications. Controllable mixers 6 down-convert the signal coming from the tuneable filters to a fixed intermediate frequency IF, which is lower than the RF frequency. Down-mixing of the received RF frequencies is necessary since the processing circuitry which is extracting the transmitted content from the modulated RF signal is not capable of directly operating in the high frequency ranges which are transmitted. Variable frequency oscillators LO1, LO2, LO3 are used to tune the various input frequencies to the desired intermediate frequency IF. In a typical receiver known from the state of the art a filter 7 with a narrow bandwidth and having a fixed centre frequency is connected to the output of the mixer 6. This filter is provided to isolate the desired signal from the group of output signals present at the output of the mixer. In FIG. 2 a surface acoustic wave filter, also referred to by its acronym SAW, provides this channel separation. The SAW filter has a fixed centre frequency which is corresponding to the intermediate frequency IF. An amplifier 8 may be provided to buffer the filtered intermediate frequency IF. The tuner shown in FIG. 2 may be adapted to receive signals from an antenna, as shown in the figure, or to receive signals from a cable network. Depending on the type of signals the tuner is designed for, the double-tuned filter may have different properties as to bandwidth and slope.
Double-tuned filters are requiring a rather high number of selected and matched components and efforts have been made, therefore, to reduce the amount of circuit outlay necessary.
To achieve a high level of image rejection, state-of-the-art tuners utilise a dual-conversion architecture having two mixers and surface acoustic wave filters 7. An exemplary block diagram of a double conversion tuner 300 according to the state of the art is shown in FIG. 3. The tuner 300 performs two frequency translations (one up-conversion, one down-conversion) to meet the high image rejection requirement. Overall, the tuner 300 down-converts a selected channel from a radio frequency signal RF, and outputs the selected channel as an intermediate frequency signal IF2. In the figure, the RF signal having multiple channels at multiple carrier frequencies is received by an antenna 1. The antenna may also be a cable network connection. The RF signal is typically fed to a band select filter 2, which is used to split a rather large range of possible RF frequencies into a number of paths each accommodating a smaller range of frequencies, as was discussed above. In FIG. 2 only one exemplary path of a tuner is shown. The signal is then fed to a variable amplifier 3. The variable amplifier 3 ensures that the following stages receive a signal at an appropriate level. A first mixer 4 up-converts the received RF signal to a first intermediate frequency IF1 that is fixed above the RF signal band, using a first, variable-frequency local oscillator signal LO. The local oscillator signal is controllable so as to obtain an up-converted signal at a fixed first intermediate frequency IF1, above the range of the input frequencies. A surface acoustic wave filter 7, centred at the first intermediate frequency IF1 and having a narrow pass band at the intermediate frequency IF1 selects a desired channel that falls within its narrow pass band. The SAW filter substantially rejects all of the remaining channels and provides the necessary image rejection to prevent signal interference. A second mixer 9, which is driven by a fixed frequency local oscillator signal LO2, down-converts the signal at the first intermediate frequency IF1 to a lower second intermediate frequency IF2. The frequency of the signal LO2 is appropriately selected to provide an IF at the desired intermediate frequency IF2. A second SAW filter 11 further removes any unwanted signals from the intermediate-frequency signal, resulting in the signal IF2. The signal IF2 is amplified by the amplifier 8, to produce the output. Channel selection is realised by adjusting the first and second local oscillator signals LO, LO2 so that the desired up-converted channel falls in the narrow pass band of the SAW filters 7, 11. The remaining channels, in particular the adjacent channels, are rejected by the SAW filters.
The dual conversion architecture of the conventional tuner 300 has several disadvantages. For instance, there are two of each component including two mixers, two high frequency local oscillators, and two SAW filters. In this tuner concept, fair image rejection is achieved at the expense of higher noise and the associated lower sensitivity due to the double conversion.
The high outlay in circuitry and the rather high number of components necessary for building double-conversion tuners and double-tuned filters in single- or direct-conversion tuners has led to efforts to reduce the complexity of the tuners.
One method is using an image reject mixer in a single conversion tuner. The image reject mixer is key to performing the down-conversion operation in a single frequency conversion, instead of the conventional dual-conversion operation. In other words, the image reject feature supplants the need for a dual conversion architecture.
FIG. 4 illustrates an image reject mixer 400. The image reject mixer 400 includes an in-phase divider 402, component mixers 404a, 404b, and a quadrature divider 406. The in-phase divider 402 receives an RF input signal RF, and divides the signal RF into component signals 403a and 403b, where the signals 403a and 403b are substantially equal phase and equal amplitude. Signals 403a and 403b are also referred to as I-component and Q-component, respectively. The quadrature divider 406 receives a local oscillator signal LO and divides the signal LO into component LO signals 405a and 405b, where the signal 405b is phase shifted by 90 degrees relative to the signal 405a. The mixer 404a mixes the I-component signal 403a with the LO signal 405a, resulting in the in-phase IF component I. The mixer 404b mixes the Q-component signal 403b with the LO signal 405b, resulting in the quadrature IF component Q. The in-phase IF component I and the quadrature IF component Q are combined by a polyphase filter (not shown). The image rejection occurs when the in-phase and quadrature components I, Q are combined because the phase relationship between I and Q components causes signal cancellation at the image frequency.
Theoretically, infinite image rejection is achievable if the I and Q channels of the mixer 400 are perfectly balanced at the frequency of interest. However, if the phase relationship between the I and Q channels varies from 90 degrees at some frequency, then the actual image rejection deteriorates at this frequency. Additionally, if the amplitude varies between the I and Q channels, then the image rejection also deteriorates. The amplitude and phase relationship between the I and Q channels is often collectively referred to as I/Q balance. Perfect I/Q balance is achieved when the amplitude response of the I and Q channels is equal over frequency, and the phase difference between the I and Q channels is 90 degrees over frequency.
In FIG. 5 a prior art tuner 500 is shown that uses an image reject mixer 400 as presented above. As was discussed above, image reject mixers provide inherent image rejection due to their principles of operation. In FIG. 5 an RF signal having multiple channels is received by an antenna 1. The signal is fed via a tuneable pre-select filter 4 and an amplifier 3 to the image-reject mixer 400. Amplifier 3 is a variable gain amplifier that ensures a proper signal level for the following processing stages. The image reject mixer 400 directly down-converts the input signal RF to an intermediate frequency signal IF1, using a local oscillator signal LO. The local oscillator is variable, responsive to a tuning signal (not shown) in order for the downmixed intermediate frequency signal IF1 to fall in the narrow pass band of a filter 7. Image reject mixers provide a relatively high level of image frequency rejection by way of operation. Since image rejection is one of the main objectives of tuner designers and developers the selectivity of the tuned filter circuit upstream of the mixer may be small. Downstream of the image reject mixer the downmixed intermediate frequency signal IF1 is fed to the filter 7, which selects the channel of interest from the IF signals. The filtered signal is further connected to an amplifier 8. Channel selection is performed by changing the frequency of the signal LO1, thereby causing the desired channel to shift into the pass band of the filter 7. The image rejection of this circuit is determined by the image rejection properties of the image rejection mixer 400 and the selectivity of the filter 4.
FIG. 6 shows another prior art tuner 600 using an image-reject mixer 400. An RF signal is received by an antenna 1 and fed via a band select filter 2 and a variable gain amplifier 3 to a variable low-pass filter 14. From the variable low-pass filter 14 the signal is fed to the image-reject mixer 400. The intermediate frequency IF1 at the output of the image-reject mixer 400 is fed via SAW filter 7 for channel separation to an amplifier 8. In the same way as for the circuit of FIG. 3 this tuner provides a fair amount of image rejection at a reasonable circuit complexity. The low pass filter prevents overdriving of the mixer by higher-frequency harmonics of the input signal. However, the low-pass filter does not reject sub-harmonic frequencies. This may cause an overdrive at the input of the image-reject mixer and/or lower frequencies being mixed into the desired output signal.
In view of what was explained above it may be stated that the known single- and double-conversion tuner concepts are not ideally adapted for performing both, off-air and cable reception. In order to compensate for the drawbacks of the different tuner concepts and the associated detrimental effects on the reception quality, state-of-the-art television receivers for off-air and cable reception use two tuners that are each optimised for off-air reception or cable network reception, respectively.